This application claims the priority of German patent document 100 62 547.9, filed Dec. 15, 2000, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to an aluminum cast alloy and to a component.
A recrystallization-hardenable aluminum alloy is known in the art from DE 44 04 420 A1 which has the following composition:
8.0 to 10.9 weight % silicon,
0.8 to 2.0 weight % magnesium,
4.0 to 5.9 weight % copper,
1.0 to 3.0 weight % nickel,
0.2 to 0.4 weight % manganese,
and less than 0.5 weight % iron.
(weight %=per cent by weight, proportion of the individual elements in the total material mass of alloy.)
This alloy is especially designed for pistons in internal combustion engines. The relatively high silicon share produces good resistance to wear and tear and high solidity even at high temperatures. The remaining alloy elements prevent sharp primary silicon crystals from forming that constitute, at alternating loads, the starting points for repeated stress failures. However, components of this type only have limited breaking elongations.
DE 42 15 160 C2 describes an aluminum alloy for pressure die casting applications that ensures ease in removing the mold of a component from the pressure die casting mold. Aside from 99.7% pure primary aluminum pig, it has the following composition:
5.0 to 12.0 weight % silicon,
0 to 0.8 weight % magnesium,
less than 0.01 weight % copper,
less than 0.2 weight % iron,
0.1 to 0.5 weight % cobalt.
In general, iron is added to the alloy to reduce the adhesion between the component and the die casting mold of the alloy; however, at higher concentrations, this increases the brittleness of the component. In this context, it is cobalt in particular that manifests the functional property of reducing the adhesion properties of the component to the die casting mold without leading to an increase in brittleness. Consequently, the iron portion can be greatly reduced.
The brittleness of the alloy, addressed previously, which is attributable to the different elements of the alloy and is acceptable for use as a compromise in various applications, will lead to failures for certain highly stressed components. This is true, in particular, with regard to engine components such as cylinder heads or cylinder crank cases. These components operate under particularly high temperatures, pressures and alternating loads. Moreover, complex geometry-specific reasons are responsible for extensive notch effects. If component failures are to be avoided, extraordinarily high ductility of the material is required in these cases. In particular, this applies with respect to modern high performance engines in which the loads on the cylinder heads are steadily increasing.
Therefore, it is an object of the present invention to provide an alloy that is suitable for producing components with thermal stability, high breaking elongation and high ductility while, simultaneously, the susceptibility to corrosion is minimal.
The alloy according to the present invention contains a silicon part of between 5% and 10%. If the silicon part were lower, it would impair the castability of the alloy. If the silicon part were higher, it would result in the embrittlement of the material. Preferably, the silicon part is between 6.5% and 7.5%.
Together with the silicon, the alloy element magnesium forms Mg2Si (magnesium silicide) crystals, thereby increasing the stability. If the magnesium part is below the lower limit according to the invention, the stability of the resulting component is too low; if the magnesium part is above 0.35%, the Mg2Si crystals cause excessive brittleness.
The alloy element nickel forms, in conjunction with aluminum, intermetallic phases, such as e.g. Al3Ni (nickel aluminide), that improve the thermal stability and do not congruently melt until temperatures of over 800xc2x0 C. are reached (in contrast to Al2Cu (copper aluminide) that forms in alloys containing copper and melts at temperature below 600xc2x0 C.). Moreover, the phases containing aluminum and nickel do not have any negative effect on the ductility of the material. The nickel part of the alloy according to the present invention is between 0.3% and 3%, preferably between 0.5% and 2.5%.
It is possible to add cobalt as an alloy element to the alloy according to the invention. Cobalt also forms intermetallic compounds on the basis of aluminum and cobalt, similar to the compounds on the basis of aluminum and nickel, thereby increasing the thermal stability. The alloy according to the invention can contain between 0.6 weight % and 3 weight % of cobalt.
Iron, which is used to reduce the breaking elongation, is not necessary for the alloy according to the invention. The same applies with regard to copper as an alloy element, which reduces the corrosion resistance.
Another objective according to the invention is a component. The component is cast from an alloy according to the present invention and has the advantages resulting from this alloy.
A thermal treatment of the component, preferably following a solution heat treatment, leads to precipitation hardening (heat treatment) of an Al-matrix (which constitutes the component) by way of calculated precipitating of intermetallic phases, such as e.g. the Mg2Si or Al3Ni. The precipitation hardening occurs within a temperature range of between 160xc2x0 C. and 240xc2x0 C. for a duration of between 0.2 hours to 10 hours. Particularly preferred is the precipitation hardening at temperatures of between 180xc2x0 C. and 220xc2x0 C. and for a duration of 0.5 hours to 8 hours. The length of the temperature treatment is dependent on the temperature. At higher temperatures, the heat treatment is considerably shorter.
The component, represented by way of the alloy according to the present invention, is preferably realized as a sand casting or permanent mold casting component since this facilitates the heat treatment referred to previously. For a component that is manufactured by way of the pressure die casting process, thermal treatment is not easily possible due to trapped air. In such cases, it would be necessary to use a vacuum pressure die casting process, which is more complex in terms of materials processing.
It is particularly useful if the component according to the present invention is realized as a cylinder head or as a cylinder crank case in an internal combustion engine. These components, especially cylinder heads, are exposed to very high pressures at high temperatures. Furthermore, the geometry of these components is highly complex, such as, for example, on the valve bars inside the cylinder head or on the cooling ducts inside the cylinder crank case. In particular at high temperatures, pressures, and alternating loads, these constructions act as notches and starting points for material failures. An especially high breaking elongation in combination with increased thermal stability offers considerable advantages.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the present invention when considered in conjunction with the accompanying drawings.